The known GSM-GPRS/EGPRS cellular system appears as the most relevant prior art on the field of the present invention, so before the disclosure of the invention the following panoramic on the system is given, taking advantage from the large GSM specifications coming from ETSI (European Telecommunications Standards Institute). Details on the constitution of the apparatuses and on the power control procedure will be given in the non-limiting embodiment.
The so-called General Packet Radio Service (GPRS) has been added to the Global System for Mobile communications (GSM) in order to achieve higher performance with data handling. This object is met by the introduction of a packet switching feature which doesn't request fixed connections for all the duration of an active session. Additional GPRS useful features for improving data handling are the capability to perform dynamic channel allocation to several users and the multislot assignment. The same packet data services also exist in Digital Cellular System (DCS) 1800 MHz.
The GSM-GPRS system bears three classes of operation for mobile stations: a class A mobile operates with GSM and GPRS simultaneously; a class B mobile watches GSM and GPRS control channels but can operate only a set of service at a time; finally a class C mobile only uses GPRS services. Furthermore physical resources at the Um interface can be shared between speech and packet data services on the basis of the traffic cell planning.
The GPRS service bears Quality of Service (QoS), see GSM 03.60, to assure among other things the following requirements: respect of a negotiated priority of service, service reliability, guarantee of a fixed end-to-end packet transfer delay, guarantee of medium and peak throughput in conformity with a certain multislot class. QoS parameters together with A, B, and C classes of operation and class of multislot capability take part in a User Profile made known to the network during GPRS attach.
GPRS exploits the same GMSK modulation (Gaussian Minimum Shift Keying, detailed in GSM 05.04) as GSM system and four convolutional Coding Schemes CS1 to CS4 to differently protect transmission bursts against transmission errors. An Enhanced GPRS (EGPRS) version made possible increased data-rates thanks to a higher modulation level, namely 8-PSK (Phase Shift Keying), in combination with additional five coding schemes. GMSK modulation is a non-linear Continuous Phase Modulation (CPM) characterized by compact spectrum and constant modulation envelope. The last feature also belongs to the EGPRS higher level modulation 8-PSK (Phase Shift Keying). Compact GMSK spectrum generates poor interferences into adjacent frequency channels by introducing a slight worsening of the intersymbolic interference. Constant modulation envelope allows the gain saturation of the power amplifier (class C amplifying) and consequent energy saving from the power supply. Besides, as far as concerns the present invention, downlink power control becomes easier to handle.
FIG. 1 of the description is similar to the FIG. 2 of standard ETSI GSM 03.60—“Service description”. The system of FIG. 1 represents a cellular GSM(DCS) and GPRS(Enhanced) network including mobile stations communicating via radio with a fixed remaining part. In FIG. 1 a first type of Mobile Stations MS is suitable for voice communication (and short messages), while second type named User Equipment UE, each comprised of a Terminal Equipment TE for handling data (i.e. a PC) connected to a Mobile Terminating equipment MT, is suitable for data packet transmission. Mobile stations MS and UE camped on a cell are connected, via standard on air interface Um, to a fixed Base Transceiver Station BTS which serves either a central or trisectorial cell belongs to a clustered geographical area covered by the GSM-GPRS Public Land Mobile Network PLMN. The amplification gain of each BTS transmitter shall be individually regulated to meet a power level of the transmitted signal in accordance with the objective of the invention that will be disclosed.
In the remaining part of FIG. 1 more base stations BTS are connected to a Base Station Controller BSC through a not fully standardized Abis interface. The BSC controller includes a block PCU (Packet Control Unit) specialized in packet handling. The BSC controller, among its various tasks, weighs up the conditions to perform uplink power control (detailed in GSM 05.08) and downlink power control adopting the criterion of the invention in so far as it concerns GPRS/EGPRS. The BSC controller and its interconnected base stations BTSs constitute a Base Station Subsystem BSS serving a cluster of cells. An BSC controller in its turn is connected to a Message Switching Centre MSC and to a Service GPRS Support Node SGSN via the standard interfaces A and Gb respectively, both supporting SS7 signalling. The MSC centre and SGSN node are connected to a Home Location Register HLR and a Visitor Location Register VLR which add intelligence to the network by allowing mobility of communications. The MSC centre and SGSN node support Short Message Service SMS, being for this purpose connected to a Short Message Service Centre SM-SC via the functions SMS-GMSC (Short Message Service—Gateway MSC) and SMS-IWMSC (SMS—InterWorking MSC). The SGSN node is further connected to: 1) another SGSN node of the same PLMN network through a standard Gn interface; 2) a Gateway GSN node GGSN belonging to another PLMN network through a standard Gp interface; 3) a Gateway GSN node GGSN belonging to the same PLMN network, through the Gn interface, and the GGSN node is connected to either an IP (Internet Protocol) network or a X.25 Public Data Network PDN both specialized in packet data routing; 4) finally to an Equipment Identity Register EIR. The MSC centre is connected to the Public Switching Telephone Network PSTN also comprised of an Integrated Services Digital Network ISDN. Other than the mentioned interfaces, also the following standard ones are provided: Gf, Gs, Gr, Gd, D, E, C whose connections are visible in FIG. 1.
The schematized GSM-GPRS system is capable to switch both the traditional voice/data circuits and the new packed data. The SGSN node has the same role for packet data as the MSC centre has for voice circuits, it traces individual locations of the mobile stations enabled for data packet communication and performs security and access control functions. For this purpose the HLR register includes information concerning GPRS users. The GGSN node provides interworking with external data packet switching networks, in particular with a backbone network based on IP protocol.
Both GSM and GPRS use standard procedures at the relevant interfaces, namely for: synchronization, cell selection and reselection, paging, access control, requesting a dedicated channel, security, error detection and correction, retransmission of errored blocks under type I or type II ARQ (Automatic Repeating reQuest), uplink and/or downlink power control, voice and data flux control, routing, handover, billing, etc. Such procedures belong to a most general protocol having a layered structure named “Transmission Plane” proposed by the International Organization for Standardization (ISO) for Open System Interconnection (OSI). Based on ISO model an OSI system can be described by means of a set of subsystems fit in a protocol stack. A subsystem N which consists of one or more entities of level N interacts only with subsystems immediately upon and below it and a level N entity operates into its own level N. Peer level N entities communicate each other by using services from the underlying layer N. Similarly, layer N services are provided to the layer N+1 at an N-Service Access Point named N-SAP. Information transferred from a starting to an arrival point is always conveyed by physical channels provided at the crossed interfaces. Relevant layers for the arguments developed in this disclosure are the following:                Radio Link Control/Medium Access Control (RLC/MAC).The RLC layer-2 function provides a radio link with reliability and maps into GSM physical channels the Link Layer Control (LLC) layer-3 frames. The MAC function is provided to control and signalling procedures for accessing radio channel, i.e. request and grant. RLC/MAC protocol is standardized in GSM 04.60.        GSM RF is pertaining to the physical radio channel at the Um interface as standardized in the series of specifications GSM 05.xx The physical channel relevant for GPRS service is named PDCH (Packet Data Channel).        
At GPRS planning stage the compatibility with pre-existent GSM system and procedures has been deliberately maintained to enable GPRS of exploiting the same physical channels as GSM at the Um interface and consequently promoting an easy integration. Both for GSM and GPRS there are signalling channels and traffic channels, the first ones are either for broadcast common control or for dedicated control, the second ones are either for voice or packet data. The additional logical GPRS channels, although referred to packet data have names and functional characteristics which follow from the conventional GSM channels; examples of relevant GPRS channels are the following: PBCCH (Packet Broadcast Control Channel), PCCCH (Packet Common Control Channel), PACCH (Packet Associated Control Channel), e PDTCH (Packet Data Traffic Channel). A list of relevant channels is reported in the specification GSM 05.01 titled “Physical layer on the radio path”.
The Extended GSM 900 system is required to operate in the following frequency bands:                880-915 MHz: mobile stations transmit uplink, base station receives;        925-960 MHz: base station transmits downlink, mobile stations receive; while DCS 1 800 system is required to operate in the following frequency bands:        1.710-1.785 MHz: mobile stations transmit uplink, base station receives;        1.805-1.880 MHz: base station transmits downlink, mobile stations receive.        
Each of the above frequency band is also used in GPRS service and includes a plurality of modulated carriers spaced 200 kHz apart. Full-duplex communications take place by Frequency Division Duplexing (FDD) technique. A carrier among those in use in a cell is assigned for all the duration of a timeslot TS out of eight cyclically repeated to allow time division among the users. During the assigned timeslot either GMSK or 8-PSK modulation impresses the characteristics of the modulating burst onto the phase of a carrier to be transmitted at radiofrequency.
With reference to FIG. 2 it can be appreciate the sequential organization of 8 timeslots TS0, . . . , TS7 constituting a 4,615 ms basic frame used in Time Division Multiple Access (TDMA) GSM-GPRS system. Four different typologies of burst are provided corresponding to the possible contents of any timeslot. The sequential frames are organized within more hierarchical levels observed by all the carriers used in the system. All the carriers transmitted by a BTS have reciprocally synchronized frames. Starting in the figure from bottom to top each timeslot has 0,577 ms duration, corresponding to 156,25×3.69 μs bit duration, and carries an information burst containing 142 useful bits, 3+3 tail bits TB, and a guard time GP without information 8,25 bits long. The 3.69 μs bit duration corresponds to 270,83 kbit/s which is the system cipher rate. The burst can be of four different types, namely: Normal burst, Frequency Correction burst, Synchronization burst, and Access burst. For the purposes of disclosure the only Normal burst is depicted in FIG. 2 where it includes 2×58 useful bits, redundancy included, and 26 bits of a training sequence in midamble position. Training sequence is a known pattern used to dynamically synchronize the received burst and to estimate the impulse response of the radio channel for correctly demodulating the incoming signal. The nature of the 116 bits payload will be detailed later on, distinguishing between GSM and GPRS. Continuing towards the upper part of FIG. 2 it can be noticed that two different typologies of multiframes are foreseen, namely a signalling multiframe for carrying control channels and a traffic multiframe for carrying payloads and associated signalling. The signalling multiframe is 253,38 ms long and includes 51 basic TDMA frames. A GSM traffic multiframe is 120 ms long and includes 26 basic TDMA frames. A GPRS traffic multiframe is 240 ms long and includes 52 basic TDMA frames. The two type of multiframes concur to form a unique superframe 6,12 seconds long, consisting of 1326 basic TDMA frames, finally 2048 sequential superframes form one iperframe of 2.715.648 basic frames TDMA of 3 h 28 m 63 s 760 ms duration. A frame Number FN referred to the frame position in the iperframe is broadcasted within the cell.
FIGS. 3a and 3b show traffic channel organization in the TDMA multiframes for voice/data and packet data respectively. FIG. 3a concerns GSM payload where a multiframe of 26 basic frame includes: 24 traffic frames (T), 1 associated control frame (A), and 1 idle frame (−). A physical channel inside a multiframe is constituted by the combination of one frequency and one repetitive timeslot. A singular burst of FIG. 2 span several periods of the RF carrier modulated by the relevant data stream. A burst therefore represents the physical content of a timeslot.
FIG. 3b concerns GPRS payload where a multiframe of 52 basic frames includes 12 radio blocks B0, . . . B11 of 4 basic frames each, intercalated with an idle frame (X) every three radio blocks. A radio block is carried on a channel defined as above spanning over 4 TDMA frames, so as the mean transmission time of a RLC block is near 20 ms.
FIG. 4 is referred to the GPRS service and shows a mapping of sequential RLC layer blocks into physical layer. Each RLC block includes a block header BH of variable length, an information field comprising data coming from the upper layer LLC, and a field Block Check Sequence BCS used for error detection. A single RLC block is mapped into 4 sequential frames of the TDMA multiframe. So until 8 users can be interleaved in the period of a radio block.
GSM's payload timeslots are allocated one to one the different users, both in uplink and downlink, while as far as concerns GPRS service a flexible allocation is available. More precisely: 1) GPRS's payload timeslots are independently allocated in uplink and/or downlink; simultaneous physical links in the two directions are not mandatory as in the pure GSM; 2) singular users can take advantage of multislot allocation; 3) each configured data packet physical channel PDCH (timeslot) can be advantageously shared among different users which access it on the basis of appropriate priority rules managed from the PCU (FIG. 1). The MAC layer of GPRS protocol has appropriate procedures for governing dynamic allocation of the resources for packet data transfer. These procedures are activated from relevant control messages provided at the various interfaces to set up or set down a connection. Temporary Block Flows (TBF) are connections set up on physical layer by the MAC procedures, they include memory buffers to accommodate the queues of RLC/MAC radio blocks. Each TBF connection allows unidirectional point-to-point transfer of user data and signalling between a mobile station and BSC, or vice versa. A TBF connection is held for the only transfer of all the RLC/MAC blocks of a LLC protocol session. The network assigns to each TBF connection a respective Temporary Flow Identity, named TFI identifier, by associating a field in the header of RLC/MAC blocks. A mobile station can have:                both a downlink and an uplink connection, in this case the mobile station shall assume that TFI identifier is unique for uplink and downlink concurrent TBFs;        either a downlink or an uplink connection solely.        
The header of RLC/MAC blocks further includes fields to specify direction and type of a control message. In case of dynamic allocation of the resources and in presence of at least one uplink TBF connection, the header of each RLC/MAC block transmitted downlink includes an Uplink State Flag field (3 bits), named USF, written from the network to enable the uplink transmission of a successive radio block on the same timeslot carrying USF. The one out M mobile stations listening a radio block in the current timeslot which matches the listen USF, is the only one enabled to transmit the successive radio block inside a block period. Scheduling downlink is performed from the network directly by transmitting the selected TFI.
Outlined Technical Problem
The outlined USF mechanism causes some problems in the performance of downlink Power Control (PC). As known, downlink PC is a BSC-BTS procedure which step by step modifies, within some range, the RF transmission power relevant to a controlled channel. The entity of the power modification depends on preceding Power and Quality measures performed by the mobile station on the received signal, and periodically transmitted towards the BTS. Power control procedure remedies for path loss and shadow attenuations; besides it further reduces the overall interference of the system, improving spectral efficiency, by reducing the transmitted power to the only level compatible with the target quality or data-rate. The problem arises because the dynamic allocation of the data packets forces at least two mobile stations to both reliable receive the same data packet (RLC/MAC Radio Block) transmitted by the BTS. Of the two mobile stations, a first one receives the payload and a second one is that out of N which matches with the transmitted USF. The two mobile stations can be wherever allocated in the cell and most probably the respective radio channels will meet with different physical conditions, nonetheless both of them have to be served for the best from the BTS station. The network, to perform this task in the right way, has to known quality and attenuation on both downlink radio channels of the two mobile stations concerned; that implies the existence of at least the two downlink TBFs (the ideal condition being concurrent TBFs for each mobile station), but this is not always assured due to the reciprocal independence of uplink from downlink TBF assignment. In fact, depending on the actual operative context, the mobile station addressee of the USF should stand up without a downlink TBF as well and cannot inform the network consequently.
In conclusion, the outlined problem arises for mobile stations with an uplink TBF only. These mobiles, if not otherwise provided, don't transmit towards the network any information on the channel condition; differently from the mobile stations having either downlink or concurrent TBFs, which transmit uplink to the network information concerning the borne interference and the power level of the listened BCCH channel relevant to the serving cell. It's useful outline that for downlink power control the network decisions should be primarily based upon the received signal quality, namely the C/I ratio, rather than on the received signal level (C_VALUE, RXLEV). The reason behind this is that the transmitter power directly affects the quality of the radio link, so that the received signal may be dominated by cochannel interference.
Defects of the Prior Art
GPRS specifications indicate a power control procedure suitable for managing the outlined case of mobile stations without the downlink TBF. In such a case the network obtains information on the radio channel conditions by activating a standard procedure named “Network Control Reselection” in the operative mode which supplies the network with measures originated from the mobiles. The absence of a downlink TBF makes de facto impossible to that mobile station addressee of the USF the performance of a systematic measure of the interference level; so that the unique measure performed is the level (RAVG—SERVINGCELL) of the listened BCCH of the serving cell, always broadcasted at the maximum power level. Information about interference is not provided at all by this procedure. The network, based on the level measure (RAVG—SERVINGCELL) received uplink, calculates the maximum level (RAVG—PDTCH) which can be reached on an hypothetical downlink traffic channel PDTCH set up for the transmission towards the mobile station addressee of the USF flag. Then it establishes a threshold (T_LEV_USF) corresponding to the desired level on the hypothetical PDTCH channel at the mobile station side. From the difference between the calculated maximum reachable level (RAVG—PDTCH) and the threshold (T_LEV_USF) a correct power reduction is obtained, spanning from zero and an allowed upper limit (MAX_PR_TBF) of 10 dB.
The drawback of the outlined downlink GPRS power control procedure is the complete absence of any countermeasures against the intrinsic unreliability of the calculated power reduction threshold (T_LEV_USF), mainly due to the unknown effect of the interference. The network is consequently unable to properly correct the entity of the power reduction in order to avoid that the USF flag be too frequently unlistened by an addressed mobile station. An unlistened USF wastes overall transmission time because the mobile addressee of the unlisten USF loses the opportunity of transmitting the next RLC block and in the meanwhile transmissions are prevented to the other mobiles. Furthermore, communication between the network and the mobile station is governed by a handshake mechanism by which the network sending USF waits for data/control information back from the mobile since the next block period, or after a fixed time (RRBP). Whether an RLC block is not returned in due time the network decides either to re-assign USF to the same MS or assign it to another MS, depending on the scheduling algorithm. Too many re-assignment denote unreliability of the uplink connection and the impossibility to comply with transmission delay or other quality targets, so the connection shall be sooner released. These drawbacks become particularly serious in those environments characterized by strong cochannel interferers such as the small urban cells.
Purposes of the Invention
The main purpose of the present invention is that to overcome the drawbacks of the prior art and indicate a power control method for properly transmitting downlink time slotted information on the air-interface of packet switching cellular networks performing dynamic allocation of the RF channel among several users scheduled to access it.